Many wireless communication protocols provide for transmitters, operating within a communication network, that are capable of transmitting at varying levels of output power. One reason for having varying level of output power is to accommodate the transmitter in wireless mobile communication devices such as wireless telephones, wireless personal data assistants (PDAs), pagers, two-way radios, and other types of wireless devices (all referred to herein as “User Equipment” or “UE”), which may be located at a varying distance from a base station.
Additionally, the increased data rates that have been enabled by modern mobile radio standards (e.g., LTE, HSPA, CDMA, WCDMA, WiMax) have placed increased demands on mobile device (UE) transmitters. While the maximum transmit power for a UE transmitter remains unchanged, the average transmit power of the UE's transmitter can be significantly higher than in the past. This increase in the average transmit power leads to significantly higher power consumption by the overall transmitter circuitry and to additional heat production from the transmitter circuitry due to inefficiencies of prior transmitter designs.
Additionally, multimode wireless devices are designed to transmit communication signals of different signal modulation schemes using a single power amplifier. Therefore, the single power amplifier must also be capable of transmitting at the power output levels required for each of the different RF modulation schemes associated with the modern radio standards (e.g., LTE, HSPA, CDMA, WCDMA, WiMax). Power amplifiers required to transmit signals of different modulation schemes are typically optimized for operation in only one of the various modulation schemes, such as non-linear modulation. As a result, when the power amplifier is amplifying a signal of another modulation scheme, such as linear modulation, the power amplifier is less efficient.
In some situations, different types of modulation schemes have been created with different techniques to provide a supply voltage to a power amplifier so as to make the power amplifier more efficient for the particular type of radio frequency (RF) modulation. One technique for providing voltage to a power amplifier (PA) is called envelope tracking (ET). ET is where the supply voltage to the PA follows or tracks the varying signal envelope of the RF signal that is to be amplified. Yet another technique for providing voltage to a PA is called average power tracking (APT). APT is where the supply voltage to the PA is adjusted based on the average power level of the varying RF signal to be amplified.
For example, when designing a PA to reduce power consumption when an envelope tracking technique is used, a typical PA design combines a standard buck converter to drive average current and a dynamic boost converter to drive envelope peaks. The maximum output voltage of the buck converter will be limited by the maximum voltage of the battery (Vbatt). Yet, the dynamic boost converter will allow the PA to drive voltage peaks (˜3 dB) above Vbatt. The dynamic boost converter that is part of an envelope tracking modulator, which supplies the voltage for envelope tracking is sized to drive signal peaks only, but is not sized to provide the power needed when average power tracking (APT) is the mode of operation. Thus, when this design is used in an APT mode, the PA will be inefficient, clip some of the output signal peaks, and produce more heat.
In general, a power amplifier that is optimized for ET has a higher saturated power capability when compared to a more conventional power amplifier that uses average power tracking or a fixed voltage supply.
Referring to FIG. 1, a general graph of an Icc to Vcc response curve for a power amplifier output is shown. The Vcc is the input voltage to the power amplifier and the Icc is the current in the output of the power amplifier. The input voltage (Vcc) to output current (Icc) curve 110 is also referred to as the power amplifier's response curve 110. The power amplifier (PA) response curve 110 shown is only one representation of a family of PA response curves that result from a single PA. For simplicity, another PA response curve 112 is also shown. Other PA response curves, not specifically shown, may have different slopes in the low Vcc range and different maximum or saturated Icc outputs at the higher Vcc input levels depending, for example, on the bias applied to the PA amplifier stage(s) within the PA.
Two other curves are shown in FIG. 1. One of the curves is an envelope tracking (ET) load line 114. The other curve is an average power tracking (APT) and/or fixed voltage load line 116. The two load lines 114 and 116 are shown to exemplify that for envelope tacking (ET) the optimum load impedance 114 of a PA is very different from the optimum load impedance for an APT or fixed supply voltage load line 116 of the same PA. As such, it is clear that when a PA is designed for optimum performance as an ET PA, the same PA will be substantially less efficient when operated in APT or fixed voltage input situations. In other words, an APT PA, a fixed voltage PA and an ET PA will each have very different impedance load lines that will affect each of their designs so as to function optimally within the transmitter portion of a mobile communication device.
Yet, it has become common in mobile communication transmitters of UEs to require that an ET transmitter be backward compatible (i.e., able to operate in a non-ET mode). This ability for an ET transmitter to be backward compatible is important, for example, in situations where the transmitter supports multiple radio access technologies such as LTE, WCDMA, and CDMA, but wherein not all modes of PA supply voltage modulation for the radio access technologies are supported or configured to be an ET mode. Additionally, there are situations wherein, for example, a transmission of an LTE channel bandwidth is too large to be supported by ET techniques, such that non-ET techniques, such as APT or fixed voltage modulation technologies need to be used by a PA to amplify and transmit the large LTE channel bandwidth.
Additionally, DC-DC converters can be designed to provide the supply voltage (Vcc) to a PA. Such DC-DC converters can be designed to support and provide voltage to an optimized ET PA, meanwhile retain ATP functionality, but there is a much increased expense and component size associated with such a design, which greatly impacts the feasibility of DC-DC supply voltage converter designs. As indicated, PAs can be designed to support both ET and non-ET modes of operation, but in the past when doing so performance and efficiency of the resulting PA has always suffered.
What is needed is a transmit power amp that can be optimized for operation with linear, non-linear, multi-state, average power tracking, enhanced power tracking or envelope tracking modes of supply voltage modulation methods so as to provide an efficient low power consumption adaptive power amplifier regardless of the RF transmit mode of operation (e.g., LTE, WCDMA, CDMA, HSPA, WiMax, etc). Additionally, what is needed is a PA design that can be optimized during operation for ET performance, APT performance, fixed voltage performance, i.e., for multiple supply modulation modes, and maintain functionality and performance at the lowest cost and physical size.